Conventionally, each of Japanese unexamined patent application publication 9-324613 (corresponding to U.S. Pat. No. 5,738,056) and Japanese unexamined patent application publication 2001-159330 (corresponding to U.S. Pat. No. 6,330,870) has disclosed a variable valve timing device driven by a hydraulic pressure. Such a variable valve timing device is configured to set a lock phase to a substantially middle point of a controllable range of the VCT phase when an internal combustion engine is stopped thereby to increase the controllable range of the valve timing (VCT phase). Specifically, when the engine is stopped, the VCT phase is locked in the intermediate lock phase. The intermediate lock phase is set to be suitable for starting the engine. Subsequently, the engine in the intermediate lock phase is started. Thereafter, as an engine rotation speed increases to increase a rotation speed of an oil pump, a hydraulic pressure increases. After completion of starting of the engine, a hydraulic pressure is increased to a proper hydraulic pressure. Thus, the lock is released, and a feedback control of the VCT phase is started. A crank angle signal and a cam angle signal are respectively outputted from a crank angle sensor and a cam angle sensor, in synchronism with an engine rotation. At this time, an actual VCT phase is calculated based on a phase difference between a signal generation time point of the crank angle signal and a signal generation time point of the cam angle signal. After the lock is released, a hydraulic pressure for driving the variable valve timing device is feedback-controlled such that the actual VCT coincides with a target phase. The target phase is set according to an engine operation state.
As described in Japanese Patent publication 3699654, a most retarded phase or a most advanced phase is set to a reference phase (0° A) when the actual VCT phase and the target VCT phase are calculated. In such a variable valve timing device having an intermediate lock mechanism, the VCT phase is supposed to be locked in the intermediate lock phase when an engine is started. Therefore, when an abnormal condition occurs, and a lock pin is not properly projected to perform a locking action in response to a lock request, startability of the engine may be impaired, and stability of an idling operation after an engine start cannot be maintained. Therefore, when such an abnormal condition occurs, and when the lock pin cannot be properly projected, such an abnormal condition needs to be immediately detected.
However, each of components of the variable valve timing device has a variation in its physical property such as a dimension and the like due to a dimensional tolerance. Such a variation may be also caused by aging. Accordingly, a relationship between the actual VCT phase and the intermediate lock phase cannot be correctly determined before completion of learning of the reference phase. Therefore, when a lock request occurs in a period before completion of the learning of the reference phase, and when an abnormal condition occurs such that the lock pin is not properly projected, such an abnormal condition was hardly detected promptly. Furthermore, the engine is started in a condition where the VCT phase is locked in the intermediate lock phase. Therefore, subsequent to an engine start, it takes time before establishing an operating condition in which learning of the reference phase (most retarded phase or most advanced phase) can be performed. Accordingly, in a period after an engine start before completion of learning of the reference phase, the VCT phase is controlled while the reference phase is unknown. Consequently, the VCT phase is uncontrollable with a sufficient accuracy.